This invention relates to information data transmission systems, and more particularly to information data transmission systems in which the series of information data are transmitted in predictively encoded form. 2. Description of the Related Art
In the art of analog transmissions such as in FM broadcasts by the communications satellite or on the ground surface, consideration of the transmission method has so far been limited to what is called "time compressed integration" (TCI). This comprises separation of the television signal into the luminance and chrominance signals, which are then time-axially compressed to be transmitted in the time-divisional multiplex form.
An example of the signal according to the TCI system or the TCI signal is shown in FIG. 1 where a color signal C.sub.W of the wide band axis in the color spatial frequency characteristic of visual angle appears for a time 1, another color signal C.sub.N of the narrow band axis perpendicular thereto in the uniform-chromaticity-scale (UCS) diagram as is well known in the art, occurs for a time 2, and the luminance signal Y takes place for a time 3. It is here assumed that the number of effective samples of each of the color signals C.sub.W and C.sub.N in one horizontal scanning period is 1/4 of that of the luminance signal Y, for the C.sub.W and C.sub.N both are time-axially compressed to 1/5, and the Y to 4/5. One of the C.sub.W and C.sub.N and the Y constitute a television signal for one horizontal period. As for the color information, the C.sub.W and C.sub.N are transmitted line-sequentially.
We now consider that such a TCI signal is to be transmitted in the form of digital data. In general, the digital transmission system uses the technique of reducing the quantity of data to be transmitted in such a way as to compress the band of transmission of the data to achieve an advantage that, for example, the transmission efficiency in the transmission path such as circuits is increased to transmit the signals economically. Particularly for transmission of video signals, as the technique of this sort, use is often made of being predictively encoded by utilizing the correlation of informations.
An example of it is the so-called the differential pulse code modulation (DPCM), which enables a picture of higher resolution to be transmitted with a reduced quantity of data. But there is a problem that if an error occurred in the DPC-modulated data to be transmitted, as to when to demodulate them, this error is allowed to propagate. Although it is usual in the digital transmission system that the error is corrected or compensated for by addition of an error correction code or error detection code, there exists occurrences of oversight to correction with a certain probability. From this reason, once an error has occurred in the data to be transmitted, the image is caused to deteriorate largely.
The general counter-measure against the propagation of such an error has been that for every prescribed number of data, the DPCM is not carried out and instead the data to be transmitted are merely pulse code (PC)-modulated. FIG. 2 shows an example of a data series after the application of such a kind of counter-measure against propagation of the error. All the sections shown in FIG.2 constitute a corresponding signal to one horizontal scanning period of the above-described TCI signal. In the drawing, the hatched portions represent the PCM data, and the other portions of white blank represent the DPCM data.
The DPCM encoder for producing the data series such as that shown in FIG. 2 is exemplified in FIG. 3 where the TCI signal after PCM enters at a terminal 11. The encoder includes a subtractor 12, a non-linear quantizing circuit 13, a representative value setting circuit 14, a delay circuit 15, and an adder 16. In operating the encoder, data which will later be demodulated by a demodulating system or decoder (not shown) is produced from the delay circuit 15 in delay by one data transmission period. This local data to be demodulated is applied to the subtractor 12 in which that data to be demodulated which is related to the DPCM data that just precedes the presently inputted DPCM data is subtracted from the latter to obtain a difference data. This difference data is then non-linearly quantized by the circuit 13 to obtain a data of which the number of bits is fewer than that of the difference data as the input. By using this data as the DPCM data to be transmitted, it is made possible to reduce the quantity of data to be transmitted. And the aforesaid DPCM data when in the representative value setting circuit 14 is made to change its number of bits to the original one so that it becomes the data of respresentative value in each quantizing step and is to be used for the local demodulation.
For every one datum inputted to the terminal 11, a pulse enters another terminal 17. Each time a counter 20 has counted N pulses, it once moves a switch 18 to a position B. Thereby, one out of N data to be transmitted is made the PCM datum.
In such a manner, the propagation of the error is stopped at a time when the PCM datum is transmitted after the prescribed number of DPCM data with inclusion of that error have been transmitted. In the following, the datum that comes to stop the propagation of the error will be called the "reset" datum.
When the TCI signal is transmitted by using that series of data which is of the form shown in FIG.2, however, the deterioration of the color signals becomes appreciable. This is because the length of propagation of the error in the reproduced picture when it occurred in those of the DPCM data which are comprised of the color signals is longer than when in the DPCM data of the luminance signal. With a view of the background that, from the very beginning, the color information is coarser than the luminance information, the deterioration of the image quality due to the propagation of the error becomes rapidly larger. Therefore, the reproduced picture gives a very unpleasant impression. Another problem is that the use of the line-sequential method for transmitting the C.sub.W and C.sub.N as shown in FIG. 1 leads to a possibility that when one of the data of the C.sub.W or C.sub.N errs, the color informations for at most two horizontal scanning periods cannot be recovered.